Last Names

I was once told by an editor that the first time you ever refer to each character in a novel, you must use both their first and last names,
especially when you are introducing a lot of characters. This seems awkward to me in some contexts and I wondered if it was a set rule. A web search revealed nothing. Thus I decided to consult what seemed
like the best source - the published novels on my shelves. I selected eight novels at random to inspect.

The results:

Water for Elephants (Sara Gruen) - 5 characters introduced in chapter 1, No last names in chapter 1, main character's last name first appears on page 7 in chapter 2.

The Book of Negros (Lawrence Hill) - Main character's last name
appears in chapter 1, no last names in chapter 2, 6 characters
introduced in chapter 2.

Every Lost Country (Steven Heighton) - 13 characters introduced in chapter 1, Last names are given for 4 of the characters.

The Outlander (Gil Adamson) - 3 characters introduced in chapter 1, Last name given for 1 of the characters.

The Book Thief (Markus Zusak) - 1 character introduced in chapter 1, No last names in chapter 1.

In the books surveyed, characters' last names are not generally utilized in introducing characters﻿. In the case of Heighton, who
included last names more frequently than any other author, he only did
so for his main characters, and not necessarily at the first mention of
those characters. Rather he seemed to find a way to put it in
unobtrusively in the case of two of the four characters.